1. Field of the Invention
This invention relates to gas turbine engines, and more particularly to engines having a coolable turbine section.
2. Description of the Prior Art
In a gas turbine engine of the type referred to above, pressurized air and fuel are burned in a combustion chamber to add thermal energy to the medium gases flowing therethrough. The effluent from the chamber comprises high temperature gases which are flowed downstream in an annular flow path to the turbine section of the engine. A limiting factor in many engine designs is the maximum temperature of the medium gases which can be tolerated in the turbine without adversely effecting the durability of the turbine components. The maximum allowable temperature of the medium gases is increased in most modern engines by cooling various regions of the turbine. In one engine form, cooling air from the compressor is flowable in a radially inward direction through the compressor drum cavity and axially rearward along the rotor shaft. The cooling air is discharged to the drum cavity through radially oriented bleed holes at the inner wall of the flow path for the medium gases in the compressor at a tangential velocity which approximates the local drum speed.
Various combinations of cooling air temperature and cooling air flow rate are employable to effect the desired cooling characteristics in the turbine. The pressure of the cooling air utilized, however, must be sufficient to overcome the frictional flow losses and the vortex pressure losses inherently generated as the air is flowed to the turbine. Accordingly, the source of the cooling air is selected at an axial location in the compressor which will satisfy the combined temperature, flow rate and pressure requirements.
In engine constructions having high speed rotors, the vortex pressure losses alone comprise the principal restriction to the flow of cooling air. As is expressed below, the vortex pressure loss increases in proportion to the square of the tangential velocity of the cooling air in the drum cavity. ##EQU1## WHERE .DELTA. P = vortex pressure loss
.rho. = density of the air PA1 V.sub.T = tangential velocity of the air PA1 r = radius PA1 V.sub.T r = K PA1 K = constant PA1 V.sub.T/r = K
The tangential velocity and hence, the vortex pressure loss is dependent upon the type of flow within the compressor drum. Free vortexing type flow is discussed in U.S. Pat. No. 2,830,751 to Quinn et al wherein, in accordance with the law of conservation of angular momentum, the tangential velocity of the air within the drum is inversely proportional to the radius.
where
In Quinn et al compressor air is flowable from the medium flow path in the radially inward direction at low engine speeds to prevent compressor surging. As the engine speed increases, the tangential velocity of the air within the drum increases to the point where the vortex pressure loss imposes a significant flow restriction on the air within the drum cavity. Upon attainment of a sufficient engine speed, the radial inflow of air is completely stopped.
While the free vortexing phenomenum has been advantageously employed in the Quinn et al anti-surging construction, the same phenomenum has a potentially disasterous effect on turbine systems which rely on internally bled air for cooling. In such a system a high level of air flow is required at high engine speeds to compensate for increased temperatures of the working medium gases in the turbine flow path. To insure adequate cooling air flow, the source of cooling air is positioned axially rearward in the compressor at a location imposing a sufficient pressure differential between the source and the cooled region of the turbine to overcome the vortex pressure loss at high engine speeds. Any rearward adjustment of the cooling air source is undesirable in that not only is the temperature of the cooling air raised to the detriment of cooling effectiveness, but also the overall engine efficiency is decreased by increasing the amount of compressor work required to raise the pressure of the cooling air to an adequate level.
Prior attempts have been made to reduce vortex pressure losses in cooling flow within the compressor drum. In two typical constructions U.S. Pat. No. 2,618,433 to Loos et al entitled "Means for Bleeding Air from Compressors" and 2,910,268 to Davies et al entitled "Axial Flow Fluid Machines," cooling air is flowed radially inward through passageways in the compressor drum. The passages conform the tangential velocity of the air flowing therethrough to the local tangential velocity of the rotor making the tangential velocity directly proportional to the radius.
referring to the vortex pressure loss formula above, it is evident that the Loos et al and Davies et al constructions offer substantially reduced resistance to flow when compared to the open drum cavity as shown in Quinn et al. The latter two constructions, however, do add substantially to the structural complexity of the rotor system without completely eliminating the vortex pressure loss.
Improved apparatus is required to increase engine performance by reducing vortex pressure losses without adding substantially to the cost, weight or structural complexity of the engine.